1. Field of the Invention
This invention relates to a low shrinkage unsaturated polyester resin composition which shows a low shrinkage and is excellent in colouring characteristics, surface smoothness and physical strength.
Recently the demand for sheet molding compounds (hereinafter referred to as SMC), bulk molding compounds (hereinafter referred to as BMC) and the like, which are produced by blending a filler, a pigment, a thickener, a fibrous reinforcing material, a curing agent, a stabilizer, a mold releasing agent and the like with an unsaturated polyester resin, as industrial materials for machine molding, has remarkably increased because they fulfil needs for saving manpower, effecting mass production, improving working environment and the like in the industry of processing reinforced plastics.
However, unsaturated polyester resins show a large shrinkage when they are cured and thus the resulting moldings tend to have cracks, looseness, warps, waves and the like.
In addition, the shrinkage upon curing causes embossments of reinforcing materials and peeling at the interface.
Furthermore, moldings of high dimensional precision can hardly be produced as the result of the shrinkage.
A method is adopted in general for improving these drawbacks in which a thermoplastic resin such as polystyrene, polyvinyl acetate, polymethyl methacrylate or the like is blended as a low shrinkage agent with the unsaturated polyester resin.
However, there are drawbacks in the conventional low shrinkage unsaturated polyester resin compositions which are prepared by blending the low shrinkage unsaturated polyester resin with the filler, the pigment, the thickener, the fibrous reinforcing agent, the curing agent, the stabilizer, the mold releasing agent and the like, these drawbacks being that the low shrinkage agent segregates upon thickening by the thickener and during storage resulting in contamination of the mold and spoiling of the surface conditions, that little effect is exerted for reducing the shrinkage, and that uniform colouring is hardly attained. As an improvement to overcome these drawbacks, there is described an unsaturated polyester resin composition, for example, in Japanese unexamined Patent Application Publication No. 28589/1977 which contains 20 to 80% by weight of an .alpha.,.beta.- ethylenically unsaturated polyester having a molecular weight of 220 to 350 based on one double bond and using at least 10% by mole of neopentyl glycol, hydrogenated bisphenol A, 2,2'-di-(4-hydroxypropoxyphenyl) propane or 2,2'-di-(4-hydroxyethoxyphenyl) propane based on the glycol component of the unsaturated polyester resin; 20 to 80% by weight of an ethylenically unsaturated monomer; and 1 to 25% by weight of a styrene series copolymer having an acid value of 3 to 40. On the other hand, there is shown in Japanese Laid-Open Patent Application Publication No. 44896/1977 a low shrinkage molding material which is prepared by compounding 100 parts by weight of an unsaturated polyester resin composed of 20 to 80 parts by weight of an unsaturated polyester and 80 to 20 parts by weight of ethylenically unsaturated monomer; 2 to 20 parts by weight of a copolymer containing carboxyl groups which copolymer is derived from (A) 2 to 15 parts by weight of polystyrene and (B) ethylene used in an amount of 70 to 98% by mole based on the total amount of the monomers; and 30 to 300 parts by weight of a filler.
However, these compositions or molding materials do not fulfill all the requirements of low shrinkage, coloring characteristics, surface smoothness and physical strength.